Numerous thermochemical cycles for the production of hydrogen have been proposed in the literature, and of these the UT-3 cycle is a leading exponent. The primary objective of such processes is to be more energy efficient than water electrolysis in converting heat to hydrogen. This paper presents a critical assessment of the actual energy efficiency that could be realised in a thermochemical cycle, taking the UT-3 cycle as the basis for the study. It is concluded that the upper efficiency for this process lies around 12%, on the basis of the lower heating value of the hydrogen product, comparable to current hydrogen production techniques using photovoltaics followed by water electrolysis. The practical upper efficiency may however be lower, even much lower, as several uncertainties exist in key aspects of the process. This in-depth analysis of one thermochemical process brings to the fore issues of whether such cycles can-on the basis of efficiency-compete with electricity generation followed by electrolysis. In view of the renewed interest in thermochemical cycles, analyses such as the one presented in this paper must be carried out and published in the open literature, so that the wider research community can realistically assess the prospects of this intriguing route to hydrogen production. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.